1. Field of the Invention
This invention relates to light emitting compound semiconductor crystals grown on a polar surface and, more particularly, to reduction or cancellation of their naturally occurring polarization-induced charges to improve emission efficiency.
2. Description of the Related Art
Most semiconductor light emitters have a double heterostructure structure that includes an active or light-generating layer grown between two cladding layers. The various layers of the double heterostructure are fabricated from more than one material. One cladding layer is n-type, which means it contains excess free electrons, and one cladding layer is p-type, which means it contains excess holes. In general, the cladding layers have larger bandgaps than the active layer. This causes injected electrons and holes to be confined within the active layer, encouraging efficient recombination of free carriers through spatial localization within the active layer to produce light. In addition, laser diode (LD) emitters also have separate light confining layers, typically comprised of a material with an even wider bandgap, surrounding a double heterostructure. Double heterostructure semiconductor devices are described in numerous publications, including O""Shea et al, Introduction to Lasers and Their Applications, Addison-Wesley Publishing Company, December 1978, pages 166-167.
In such structures, polarization-induced charges occur when the material composition varies in a polar direction of its basic crystal structure. A polar direction is defined as any crystal direction not orthogonal to the polarization vector, {overscore (P)}, of the crystal. This is especially true for materials whose crystal bonds are naturally directional and even slightly ionic, such as in III-V or II-VI semiconductors. These charges can be strain-related (piezoelectric) in the case of lattice mismatched materials, composition related (spontaneous) due to differences in the ionic strengths of bonds in different materials, or a combination of the two. The induced charges cause electric fields or potential gradients that have the same effect on free carriers as external fields. The phenomenon is discussed in a number of publications, including Bernardini et al, xe2x80x9cSpontaneous polarization and piezoelectric constants of III-V nitrides,xe2x80x9d American Physical Society Journal, Physics Review B, Vol. 56, No. 16, 1997, pages R10 024 to R10 027, and Takeuchi et al, xe2x80x9cQuantum-Confined Stark Effect due to Piezoelectric Fields in GaInN Strained Quantum Wells,xe2x80x9d Japanese Journal of Applied Physics, Vol. 36, Part 2, No. 4, 1997, pages L382-L385. The magnitudes of such fields have been estimated to be as high as 2.5xc3x97106 V/cm for nitride heterostructures grown on a polar surface of a crystal, Bykhovski et al., xe2x80x9cElastic strain relaxation and piezo-effect in GaNxe2x80x94AlN, Ganxe2x80x94AlGaN and GaNxe2x80x94InGaN superlatticesxe2x80x9d, Journal of Applied Physics, Vol. 81, No. 9, 1997, pages 6332-6338.
Polarization-induced charges should be taken into account when considering the electrical characteristics of heterostructures grown on crystal polar surfaces. Crystal layers grown along the 0001 orientation in the case of wurtzite GaN crystal, or along the 111 orientation in the case of zincblende GaAs crystals, are two examples of crystal polar surfaces. The Bravais lattice of the wurtzite structure is hexagonal, with the axis perpendicular to the hexagons usually labeled as the c-axis or the 0001 orientation. Along this axis the structure can be thought of as a sequence of atomic layers of the same element (e.g. all Gallium or all Nitrogen) built up from regular hexagons. Due to this uniformity, each layer (or surface) is polarized and possesses either a positive or a negative charge, generating a dipole across the atomic layers. The charge state of each layer depends upon its constituent atoms. Other examples of crystal planes with various growth directions may be found in Streetman, Solid State Electronic Devices, 2nd ed., Prentice-Hall, Inc., 1980, pages 1-24, and Shuji Nakamura et al, xe2x80x9cThe Blue Laser Diode, GaN Based Light Emitters and Lasers,xe2x80x9d Springer, 1997, pages 21-24.
Until recently, internal polarization fields associated with the active and cladding regions of a light emitting heterostructure have not posed significant problems. This was because light emitting diodes (LEDs) based on the more established Alxe2x80x94Gaxe2x80x94Inxe2x80x94Asxe2x80x94P material system have typically been grown on a non-polar crystal surface (in particular the 001 zincblende surface). Recently, however, there has been considerable work in light emitters based on the Alxe2x80x94Gaxe2x80x94Inxe2x80x94N (xe2x80x9cnitridexe2x80x9d) materials system, mostly grown along the 0001 orientation of wurtzite crystal, which is a highly polar surface. Nevertheless, nitride double heterostructures have followed conventional non-polar designs.
FIG. 1A is a sectional view schematically illustrating a typical conventional nitride double heterostructure semiconductor grown in a polar direction. The illustrated substrate layer 1 may be any material suitable for growing nitride semiconductors, including spinel (MgAl2O4), sapphire (Al2O3), SiC (including 6H, 4H, and 3C), ZnS, ZnO, GaAs, AlN and GaN. The substrate thickness typically ranges from 100 xcexcm to 1 mm. A buffer layer 2 on the substrate 1 can be formed of AlN, GaN, AlGaN, InGaN or the like. The buffer layer facilitates possible lattice mismatches between the substrate 1 and an overlying conductive contact layer 3. However, the buffer layer 2 may be omitted if the substrate has a lattice constant approximately equal to that of the nitride semiconductor. The buffer layer 2 may also be omitted with some nitride growth techniques. Depending upon the material composition, the buffer layer energy bandgap may range from 2.1 eV to 6.2 eV, with a thickness of about 0.5 xcexcm to 10 xcexcm.
The n-type contact layer 3 is also typically formed from a nitride semiconductor, preferably GaN or InGaN with a thickness ranging from 0.5 xcexcm to 5.0 xcexcm, and a bandgap of approximately 3.4 eV for GaN and less for InGaN (depending upon the Indium concentration). A lower n-type or undoped cladding layer 4 on the conductive layer 3 conventionally comprises GaN or AlGaN, with a bandgap of 3.4 eV for GaN and greater for AlGaN (depending upon the Al concentration). Its thickness can range from 1 nm to 100 nm.
Nitride double heterostructures typically employ InGaN as an active region 5 over the lower cladding layer, with a thickness of 1 nm to 100 nm. The bandgap of this layer is typically 2.0 eV, but may vary depending upon the Indium concentration. A top p-type or undoped cladding layer 6 over the active region is generally comprised of AlGaN or GaN, with a thickness and bandgap energy similar to that of the lower n-type cladding layer 4. A p-type GaN conductive contact layer 7 on the cladding layer 6 has an energy bandgap of about 3.4 eV and a thickness of about 10 nm to 500 nm. In general, provided the structure is grown on a polar direction such as the 0001, a polarization-induced sheet charge occurs at the interface between layers due to different constituent materials. Of particular concern for the operation of a light emitter are the polarization-induced charge sheets adjacent to the active region 5.
With the compound semiconductor illustrated in FIG. 1A, a negative polarization-induced charge sheet density "sgr"1, with a magnitude such as 1013 electrons/cm2, is typically formed at the interface between the active region 5 and the lower cladding layer 4. A positive charge sheet density "sgr"2 of similar magnitude is formed at the interface between the active region 5 and the upper cladding layer 6. The polarities of these charges depend upon the bonds of the crystal layers, which as mentioned above are directional and slightly ionic. In general, the density of a charge sheet will depend upon both a spontaneous factor arising from compositional differences between the two layers, and a piezoelectric strain arising from the lattice mismatch between the layers. For example, "sgr"1 between an In0.2Ga0.8N active region 5 and a GaN cladding layer 4 is about 8.3xc3x971012 electrons/cm2. This is due to the 20% Indium content in the In0.2Ga0.8N active region (spontaneous polarization), and the strain in that layer arising from the lattice mismatch with the underlying GaN layer (piezoelectric polarization).
Interfacial charge sheets along opposite surfaces of the active region produce a dipole across the region. This dipole corresponds to an electric field whose strength depends on the magnitude of the sheet charges "sgr"1 and "sgr"2. For the case given above, a sheet charge of 8.3xc3x971012 cmxe2x88x922 gives an electric field of 1.5xc3x97106 V/cm. Based on its origin, we will refer to this electric field as a polarization-induced field. The magnitude of the electrostatic potential drop generated by the dipole depends upon the thickness of the dipole layer. The thickness of the dipole layer refers to its physical dimension in the direction of growth, which is also the distance between "sgr"1 and "sgr"2. This distance can be used to determine the magnitude of the electrostatic potential drop in a manner similar to the determination of a capacitive potential drop from the distance between two capacitor plates. A distance of 10 nm between charge densities "sgr"1 and "sgr"2 as given above would result in a polarization-induced potential drop of about 1.5V across the active region 5. The net electric field across the active region also depends on a number of parameters including the doping concentration in the surrounding cladding layers, the built in voltage across the p-n junction and free carrier screening, and is therefore not generally equal to the polarization induced field. However, due to its strength, the polarization-induced field plays a major role in determining the net electric field.
Nitride emitters grown on a 0001 (polar) surface of a crystal have a low emission efficiency of about 1% to 10%. This can be due to the presence of significant polarization fields in or adjacent to their active regions that limit their efficiency. FIG. 1B illustrates the energy bands corresponding to the device structure of FIG. 1A. When the device is operating, the naturally occurring polarization field generated by "sgr"1 and "sgr"2 reduces the efficiency in a number of ways. First, the dipole leads to a spatial separation (movement in the opposite direction) of electrons and holes within the region. As illustrated, holes in the valence band Ev are attracted to the negative charge sheet "sgr"1 at one end of the active region 5, while electrons in the conduction band Ec are attracted to the positive charge sheet "sgr"2 at its other end. This spatial separation of free carriers lowers the probability of radiative recombination, reducing emission efficiency. Second, the energy barriers of the conduction and valence band quantum wells are reduced by quantization effects associated with the electric field. Thus, carriers below Ev and above Ec escape the well through the paths indicated by dashed lines A. Third, the presence of polarization-induced fields also leads to carrier overshoots, illustrated by carrier trajectories B, from the higher Ec level on the "sgr"1 side of the active region to the lower Ec level on the "sgr"2 side, and from the lower Ev level on the "sgr"2 side of the active region to the higher Ev level on the "sgr"1 side.
Another issue of concern for applications engineers is the stability of the emission wavelength as the applied bias is increased. If strong polarization-induced fields are present, the emission wavelength will blue-shift as the device bias is increased. As the device bias is increased, mote free carriers accumulate in the conduction and valence band wells. Since the free carriers are spatially separated, they will themselves form a dipole that opposes, or screens, the built-in polarization induced field. As the net electric field is reduced, the quantization states of the quantum wells change, resulting in a blue-shift of the emission wavelength.
FIG. 1C illustrates the energy bands of the active layer 5 and the cladding layers 4 and 6 for a light emitter operating on a non-polar surface with no polarizationinduced charges. All else being equal, its emission efficiency is higher since the three effects discussed above are either absent or greatly reduced.
Several approaches to increase GaN-based LED efficiency have been used. U.S. Pat. Nos. 5,959,307 and 5,578,839, both to Nakamura et al, discuss the addition of Aluminum to the cladding layers to increase the active region barrier heights for a more efficient confinement of free carriers. This addition, however, also changes the material composition of the cladding layers from GaN to AlGaN, which act to increase both spontaneous and piezoelectric polarization fields. The presence of 15% Aluminum in an Al0.15Ga0.85N cladding layer could double the polarization field in the emission layer to about 3xc3x97106 V/cm. Such fields may reduce carrier confinement and increase the spatial separation of carriers by changing the energy bands of the light emitter, thereby lowering its radiative efficiency.
The present invention seeks to improve the operating efficiency of a compound semiconductor LED, with layers grown along a polar direction by: reducing or canceling the effect of the crystal""s naturally occurring polarization-induced charges to improve carrier confinement, reducing their spatial separation, and reducing carrier overshoot.
In one embodiment, these charges are lowered by reducing differences in the material compositions of the crystal layers adjacent to the active region. The cladding layers can also be composed of a combination of elements, each of which tend to cancel the polarization effects of the others.
One or more layers in or around the active region can also be graded in composition or doping to generate space charges that oppose the polarization-induced charges, and quasi-fields that oppose polarization-induced fields generated by the polarization-induced charges. The grading may be continuous or discrete.
The compound semiconductor crystal can also have a multilayer emission system consisting of alternating light emitting and non-emitting layers to reduce the average polarization field while improving emission efficiency. The average field in the multilayer emission system as a whole is reduced or canceled compared to a single, uniform active region of comparable thickness.
Various impurities can be incorporated into the crystal that ionize, based upon their energy levels, into a charge state opposite to polarization-induced charges to reduce or cancel their effect. The impurities preferably comprise group II, IV, or VI elements.
The sign of the polarization induced charges can also be inverted to encourage, rather than oppose, the efficient confinement of carriers. These charges are inverted by inverting the atomic layer sequence of the crystal layer. The direction from which the carriers are injected can also be inverted, by inverting the growth order of p and n type layers, to screen polarization induced charges. The lattice constant of the lower buffer layer, contact layer, or cladding layer can also be changed by epitaxial growth techniques to more closely match the lattice constant of the active region. This reduces the strain-induced piezoelectric effect within the active region, reducing the polarization-induced fields for more efficient light emission.